This invention relates to a temperature function integrator and in particular but not solely to such an instrument for use in providing an indication of the deterioration of stored material.
It is well known that food stuffs in particular and in addition many other materials, deteriorate in storage and that the rate of deterioration is less at lower temperatures. (Olley and Ratkowsky, "Temperature function integration and its Importance in the Storage and Distribution of Flesh Foods above Freezing Point", Food Technology in Australia, Vol. 25 No. 2, February 1973, pp 66-73). This is particularly true of fish. Three different temperature zones are recognized for the storage of fish, see FIG. 1. At temperatures above -1.degree. C and below ambient fish is termed chilled and no water in the fish tissue is in the form of ice crystals. At low temperatures about 31 18.degree. C the fish is termed frozen and most of the water in fish tissue is in the form of ice crystals with the result that the concentration of solutes in the liquid phase prevents the growth of spoilage bacteria. In the course of freezing, the physical effects of ice crystal growth normally occur from the time the tissue is at -1.degree. until the tissue is at -5.degree. C. The temperatures zone -1.degree. to -5.degree. C, when the tissue is partially frozen, is sometimes referred to as superchilled. (Kreuzer (ed.) Freezing and Irradiation of Fish (1969) Fishing News (Books) Ltd., London pp 101-127.). The super-chilled region is always encountered by frozen fish, is sometimes encountered accidentally in the storage of chilled fish and is sometimes used to store fish for longer periods than can be used with the chilled region. It is common for fishing vessels to be equipped with fish holds cooled by ice or other forms of refrigeration which can achieve a combination of chilling, superchilling and freezing. (Nixon, Paper delivered by P A Nixon to the New Zealand Fishing Industry Board's seminar on Quality in Fish Products, Wellington, Aug. 17 - 18, 1971.). For each temperature zone, chilled, superchilled, and frozen, there are relations linking the rate of deterioration or the useful storage life to the temperature of storage. For instance, a fish which will remain edible for about 1 day at 20.degree. C will keep for about 10 days at 0.degree. C and will keep for about 100 days when deep frozen at -20.degree. C.
In order to investigate, to control or to assess the effects of the storage conditions of fish and the effectiveness of various methods of refrigeration, it is clearly not sufficient to measure only the time of storage or even the average temperature of storage. Known devices for integrating the average temperature of a material with time give information of only limited use. It is necessary to take into account the length of time that a fish has been held at any particular temperature and it is desirable to know the extent of deterioration which has occurred in the chilled, superchilled, and frozen temperature zones respectively. This can be done by recording the temperature of fish on a chart recorder or by other means multiplying the indication of temperature by a factor which embodies the rate of deterioration at the temperature in question, preparing a new record of modified temperature against time and commonly integrating over the time of storage. Such a procedure has been recommended for assessing the quality of stored frozen foods (Recommendations for the Processing and Handling of Frozen Foods; International Institute of Refrigeration 1964, pp 50-52.) This is a very time consuming process and one that is impractical to apply in most circumstances. In practice it is complicated by the fact that it is now known that the temperature indicators normally fitted in fishing vessels to indicate the temperature of the fish hold do not give the required information. It is reported by Nixon (1.c.) that the temperature of the air in the fish hold is by no means identical with the temperature of the fish.